Lithography is widely recognized as one of the key steps in the manufacture of integrated circuits (ICs) and other devices and/or structures. However, as the dimensions of features made using lithography become smaller, lithography is becoming a more critical factor for enabling miniature IC or other devices and/or structures to be manufactured.
A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of ICs. In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned.
Current lithography systems project mask pattern features that are extremely small. Dust or extraneous particulate matter appearing on the surface of the reticle can adversely affect the resulting product. Any particulate matter that deposits on the reticle before or during a lithographic process is likely to distort features in the pattern being projected onto a substrate. Therefore, the smaller the feature size, the smaller the size of particles critical to eliminate from the reticle.
A pellicle is often used with a reticle. A pellicle is a thin transparent layer that may be stretched over a frame above the surface of a reticle. Pellicles are used to block particles from reaching the patterned side of a reticle surface. Although particles on the pellicle surface are out of the focal plane and should not form an image on the wafer being exposed, it is still preferable to keep the pellicle surfaces as particle-free as possible.
A theoretical estimate of the limits of pattern printing can be given by the Rayleigh criterion for resolution as shown in equation (1):
                    CD        =                              k            1                    *                      λ                          NA              PS                                                          (        1        )            where λ is the wavelength of the radiation used, NAPS is the numerical aperture of the projection system used to print the pattern, k1 is a process dependent adjustment factor, also called the Rayleigh constant, and CD is the feature size (or critical dimension) of the printed feature. It follows from equation (1) that reduction of the minimum printable size of features can be obtained in three ways: by shortening the exposure wavelength λ, by increasing the numerical aperture NAPS or by decreasing the value of k1.
In order to shorten the exposure wavelength and, thus, reduce the minimum printable size, it has been proposed to use an extreme ultraviolet (EUV) radiation source. EUV radiation sources are typically configured to output a radiation wavelengths of around 5-20 nm, for example, 13.5 nm or about 13 nm. Thus, EUV radiation sources may constitute a significant step toward achieving small features printing. Such radiation is termed extreme ultraviolet or soft x-ray, and possible sources include, for example, laser-produced plasma sources, discharge plasma sources, or synchrotron radiation from electron storage rings.
For certain types of lithography (e.g., most extreme ultraviolet (EUV) lithography processes), however, pellicles are not used. When reticles are not covered, they are prone to particle contamination, which may cause defects in a lithographic process. Particles on EUV reticles are one of the main sources, of imaging defects. An EUV reticle (or other reticle for which no pellicle is employed) is likely to be subjected to organic and inorganic particle contamination. Particle sizes as small as around 20 nm could lead to fatal defects on the wafer and to zero yield.
Inspection and cleaning of an EUV reticle before moving the reticle to an exposure position can be an important aspect of a reticle handling process. Reticles are typically cleaned when contamination is suspected, as a result of inspection, or on the basis of historical statistics.
Reticles are typically inspected with optical techniques. However, a pattern scatters light exactly in the same way as a particle does. The pattern of a reticle surface is arbitrary (i.e. non-periodic), and so there is no way to distinguish a particle from the pattern by simply analyzing the scattered light. A reference is always required with these optical techniques, either die-to-die, or die-to-database. Moreover, existing inspection tools are expensive and relatively slow.
It has been proposed to use the presence or absence of a photoluminescence (PL) signal as an indicator of the presence of a defect, see for example JP 2007/258567 or JP 11304717. However, improvements to the particle detection capabilities of these techniques would be welcomed.